This project aims to study the genetic control of arsenic (As) homeostasis in plants. This will enable the development of plants that can selectively exclude As from their tissues, preventing As accumulation in food crops and reducing human dietary intake of As. Arsenic is one of the primary metal(loid)s of concern at Superfund Sites and chronic low-dose exposure is linked to an increased incidence of bladder cancer. Dietary studies of As intake in humans show that after drinking water, white rice is the most significant source of inorganic As for humans. A market basket survey found higher As concentrations in U.S. grown rice than rice grown in the As-affected regions of the Bengal Delta. In U.S. rice, As is thought to have come from arsenical pesticides used in the production of cotton, but As input to soil comes from a variety of industrial sources. We aim to use Arabidopsis, rice and the As-hyperaccumulating brake fern as model plant systems. They represent two species with a completed genomic sequence;one of the most important staple food crops plants and an important dietary source of inorganic As for humans, and one of the few plant species with intrinsic As resistance. We propose to use an interdisciplinary approach that combines ionomic survey techniques, quantitative trait loci (QTL) mapping and spatially resolved metal(loid) analysis and speciation via synchrotron x-ray microprobe (SXRM). The research strategy consists of gene discovery and gene characterization phases. For gene discovery, approaches include mining an existing dataset of elemental profiles of 4,000 yeast and 62,000 Arabidopsis samples for those with altered As phenotypes as well as examining natural accessions of Arabdiospis for differences in As accumulation. We will use highthroughput elemental analysis and DMA microarray-based mapping to identify genes that regulate As accumulation in rice, screening 1,790 rice accessions with the USDA's Rice Core Collection and examining QTLs for As in the Lemont X Teqing mapping population. We will use SXRM to investigate changes in the micron-scale metal(loid) distribution, abundance and/or speciation in plant tissue resulting from the deletion or silencing of selected genes of interest. This technique allowed successful characterization of gene function in a recent study of iron homeostasis. An important product of the gene characterization phase will be the online publication of an Elemental Atlas of Arabidopsis available to the wider scientific community.This proposed research expands the application of x-ray techniques beyond a spatially-resolved analytical technique into a tool for functionally characterizing ion homeostasis genes, as well as protecting human food supplies.